An ignition plug is attached to an internal combustion engine, and it is configured that a gas mixture existing in a combustion chamber of the internal combustion engine can be ignited by generating a discharge spark in a spark discharge gap of the ignition plug.
When attaching the ignition plug to the internal combustion engine (a cylinder head thereof), an insulator included in the spark plug may be broken if a bending moment is applied to the insulator.
As a countermeasure to this problem, in a spark plug disclosed in Japanese Patent No. 3711221 (refer to as Patent Document 1), when defining that an outer diameter of an insulator is D at a position corresponding to a leading edge of a terminal metal fitting inserted into a through hole formed in the insulator (an intermediate body portion), and an inner diameter of the through hole in the intermediate body portion is d, the thickness of the intermediate body portion is configured such that 0.42≤(D−d)/D≤0.79 is satisfied.
As a result, the durability against bending and impact of the insulator is remarkably improved, so that even when attaching the spark plug to the internal combustion engine, problems such as breakage of the insulator can be made less likely to occur.
Incidentally, a demand for downsizing of the spark plug is increasing in recent years. In order to downsize the spark plug, it is necessary to reduce the diameter of the insulator.
When the diameter of the insulator is reduced, it is difficult to secure the thickness at the intermediate body portion of the insulator, and it is not easy to improve the strength against breakage of the insulator.